For the purpose of automatically welding objects of welding by the arc-welding method with a consumable or a non-consumable welding electrode, along the line of a groove formed between the objects of welding, an arc-welding method is known which comprises reciprocally moving the welding electrode in the width direction of the groove while maintaining a constant prescribed distance between the tip of the welding electrode and the groove face, and at the same time, continuously moving the welding electrode along the line of the groove.
In the above-mentioned arc-welding method, the most important point is that, even when the welding direction varies from one moment to the next, the center of amplitude of reciprocation of the welding electrode always agrees with the width center of the groove.
If the center of amplitude of reciprocation of the welding electrode does not agree with the width center of the groove, there occurs a difference in the amount of penetration between one object of welding and the other, and a uniform weld bead cannot be formed.
As a welding method which enables to make the center of amplitude of reciprocation of the welding electrode always agree with the width center of the groove even when the welding direction varies from one moment to the next, we proposed an arc-welding method as disclosed in Japanese patent provisional publication No. 26,261/79 dated Feb. 27, 1979 (hereinafter referred to as the "prior art").
The above-mentioned prior art is described in detail with reference to FIG. 1. In FIG. 1, 1' is an arc current detector; 2' is a low-pass filter which eliminates detrimental noise current from values (I) of the arc current continuously detected by the arc current detector 1'; 3' is a reference value setter which previously sets in a first differential amplifier described later, a reference value (Io) for the values (I), which corresponds to the arc current value at the moment when a welding electrode 4 inserted into a welding torch 4' is located at the width center of a groove 6 formed between objects of welding 5 and 5'; 7' is a first differential amplifier which continuously calculates, for each one reciprocation of the welding electrode 4 in the width direction of the groove 6, deviations (I-Io) of the values (I) of arc current which from the detrimental noise current has been eliminated by the low-pass fitter 2', from the reference value (Io); 8' is a changeover switch which switches over the deviations (I-Io) between left-side deviations (L) and right-side deviations (R) relative to the vertical plane which passes through the center of amplitude of each one reciprocation of the welding electrode 4 and is parallel to the line of the groove 6; 9' is a pulse signal generator which generates pulse signals for switching over the changeover switch 8' and pulse signals respectively for setting and resetting a left-side deviation integrator, a right-side deviation integrator, a left-side deviation memory and a right-side deviation memory described later; 10 is a left-side deviation integrator (hereinafter referred to as the "L-side integrator") which integrates the left-side deviations (L) switched over by the changeover switch 8'; 11 is a right-side deviation integrator (hereinafter referred to as the "R-side integrator") which integrates the right-side deviations (R) switched over by the changeover switch 8'; 12 is a left-side deviation memory (hereinafter referred to as the "L-side memory") which stores the integral values of left-side deviations (L) integrated by the L-side integrator 10 as the representative value (L') of the left-side deviations (L); 13 is a right-side deviation memory (hereinafter referred to as the "R-side memory") which stores the integral value of the right-side deviation (R) integrated by the R-side integrator 11 as the representative value (R') of the right-side deviations (R); 14' is a second differential amplifier which calculates, for each passage of the welding electrode 4 through the center of reciprocation thereof, the difference (L'-R') between the representative value (L) stored in the L-side memory 12 and the representative value (R) stored in the R-side memory 13; 15' is a motor drive controller which controllably drives a motor described later so that the difference (L'-R') becomes null; and, 16' is a motor which controllably moves the welding torch 4' in the width direction of the groove 6 on the basis of a driving signal from the motor drive controller 15', so that the center of amplitude of reciprocation of the welding electrode 4 agrees with the width center of the groove 6.
Now, the case in which, when the center of reciprocation amplitude of the welding electrode 4 deviates to the left relative to the width center of the groove 6, how the position of the welding electrode 4 is corrected so that the center of reciprocation amplitude of the welding electrode 4 agrees with the width center of the groove 6 by the above-mentioned prior art is described with reference to the signal wave form diagram of FIG. 2. In FIG. 2, the signal wave forms A to F correspond to the signal wave forms A to F in FIG. 1. First, the arc current detector 1' continuously detects the values (I) of arc current. The low-pass filter 2' eliminates detrimental noise current from the values (I). The first differential amplifier 7' continuously calculates, for each one reciprocation of the welding electrode 4 in the width direction of the groove 6, deviations (I-Io) (refer to A in FIG. 2) of the values (I) from the reference value (Io) previously set by the reference value setter 3'. The changeover switch 8' switches over the deviations (I-Io) alternately between the left-side deviations (L) and the right-side deviations (R) relative to the vertical plane which passes through the center of reciprocation amplitude of the welding electrode 4 and is parallel to the line of the groove 6, by means of pulse signals from the pulse signal generator 9'. The L-side integrator 10 integrates the left-side deviations (L.sub.1). Immediately after the completion of integration of the left-side deviations (L.sub.1), the L-side integrator 10 is reset, and after the lapse of a prescribed period of time, is set again to integrate the next left-side deviations (L.sub.2). The L-side integrator 10 is reset immediately thereafter, and is set again after the lapse of a prescribed period of time. The L-side integrator 10 repeats the above-mentioned operation, based on pulse signals from the pulse signal generator 9' (refer to B in FIG. 2). The R-side integrator 11 integrates, on the other hand, the right-side deviations (R.sub.1). Immediately after the completion of integration of the right-side deviations (R.sub.1), the R-side integrator 11 is reset, and after the lapse of a prescribed period of time, is set again to integrate the next right-side deviations (L.sub.2). The R-side integrator 11 is reset immediately thereafter, and is set again after the lapse of a prescribed period of time. The R-side integrator 11 repeats the above-mentioned operation, based on pulse signals from the pulse signal generator 9' (refer to C in FIG. 2). The L-side memory 12 stores, for a period of up to the completion of integration of the next left-side deviations (L.sub.2), the integral value of the left-side deviations (L.sub.1) integrated by the L-side integrator 10 as the representative value (L'.sub.1) of the left-side deviations (L.sub.1). The L-side memory 12 is reset immediately thereafter, and then, immediately set to store, for a period of up to the completion of integration of the further next left-side deviations (L.sub.3), the integral value of the next left-side deviations (L.sub.2) as the representative value (L'.sub.2) of the next left-side deviations (L.sub.2). The L-side memory 12 repeats the above-mentioned operation, based on pulse signals from the pulse signal generator 9' (refer to D in FIG. 2). The R-side memory 13 stores, on the other hand, for a period of up to the completion of integration of the next right-side deviations (R.sub.2), the integral value of the right-side deviations (R.sub.1) integrated by the R-side integrator 11 as the representative value (R'.sub.1) of the right-side deviations (R.sub.1). The R-side memory 13 is reset immediately thereafter, and then, immediately set to store, for a period of up to the completion of integration of the further next right-side deviations (R.sub.3), the integral value of the next right-side deviations (R.sub.2) as the representative value (R'.sub.2) of the next right-side deviations (R.sub.2). The R-side memory 13 repeats the above-mentioned operation, based on pulse signal from the pulse signal generator 9' (refer to E in FIG. 2. The second differential amplifier 14' calculates, upon the completion of integration of the right-side deviations (R.sub.1), the difference (L'.sub.1 -R'.sub.1) between the representative value (L'.sub.1) of the left-side deviations (L.sub.1) stored in the L-side memory 12 and the representative value (R'.sub.1) of right-side deviations (R.sub.1) stored in the R-side memory 13, and then, calculates the difference (L'.sub.2 -R'.sub.1) between the representative value (L'.sub.2) of the left-side deviations (L.sub.2) stored in the L-side memory 12 and the representative value (R'.sub.1) of the right-side deviations (R.sub.1) stored in the R-side memory 13 (refer to F of FIG. 2). Similarly, the second differential amplifier 14' calculates the differences (L'.sub.2 -R'.sub.2) and (L'.sub.3 -R'.sub.2). The motor drive controller 15' drives the motor 16' so that the differences (L'.sub.1 -R'.sub.1), (L'.sub.2 -R'.sub.1), (L'.sub.2 -R'.sub.2) and (L'.sub.3 -R'.sub.2) become null for each calculation of these differences, to move the welding electrode 4 in the width direction of the groove 6, whereby the amplitude center of the welding electrode 4 finally agrees with the width center of the groove 6.
According to the prior art described above, it is possible to always align the center of amplitude of reciprocation of the welding electrode with the width center of the groove even when the welding direction varies from a moment to the next.
The above-mentioned prior art involves however the following problem. As is clear from FIG. 2, the above-mentioned difference (L'.sub.1 -R'.sub.1) is obtained, for example, by calculating the difference between the representative value (L'.sub.1) of left-side deviations (L.sub.1) stored in the L-side memory 12, and the representative value (R'.sub.1) of right-side deviations (R.sub.1) stored in the R-side memory 13. The representative value (R'.sub.1) is however stored in the R-side memory 13 only after the completion of integration of right-side deviations (R.sub.1). The difference (L'.sub.2 -R'.sub.1) is similarly obtained by calculating the difference between the representative value (L'.sub.2) of the next left-side deviations (L.sub.2) stored in the L-side memory 12, and the representative value (R'.sub.1) of the right-side deviations (R.sub.1) stored in the R-side memory 13. The representative value (L'.sub.2) is however stored in the L-side memory 12 only after the completion of integration of the left-side deviations (L.sub.2). This means that the position of the center of reciprocation of the welding electrode 4 is corrected always with a certain lag, resulting in a lower response of the welding electrode 4. This is particularly problematic when carrying out high-speed welding, hindering formation of a uniform weld bead.
Under such circumstances, there is a demand for a method for controlling the position of a welding electrode in arc-welding with weaving which gives a high response of the welding electrode and always permits alignment of the reciprocation center of the welding electrode with the width center of the groove even when carrying out high-speed welding, but such a method is not as yet proposed.